In numerous applications, apart from rotational speed or velocity measurements of an indicator object, often also the recognition of the direction of movement or rotational direction of the indicator object, such as the rotational direction of a wheel or a shaft, is demanded. For this, in general, magnetic field sensors are used for the determination of the rotational speed and the rotational direction.
A first possibility known in the prior art for rotational direction and rotational speed determination now consists in using two magnetic field probes arranged spatially separated from each other and spaced to the indicator object to be examined. The sensor signals of the magnetic field sensors are evaluated separately here, with the movement or rotational direction of the indicator object being determined from the temporal sequence of the signals of the magnetic field sensors for example by means of (digital) signal processing means (DSP=digital signal processor) or microprocessors. In such an arrangement, it is required to use a so-called backbias magnet in connection with the indicator object, e.g. a gear, to generate a magnetic field influenced by the various teeth and depressions of the gear, so that the two spaced magnetic field sensors can provide different sensor signals depending on the rotational position of the gear.
In FIG. 9, a schematic illustration of such a Hall sensor arrangement 10 with two Hall elements 12, 14 and a backbias magnet 16 is exemplarily illustrated with reference to a gear (or gear rack) 18. The distance L (e.g. 2.5 mm) indicates the distance between the Hall sensor IC 10 (IC=integrated circuit) and the gear 18, the distance a indicates the mean distance of the Hall elements 12, 14, and the distance b indicates the distance of the Hall elements 12, 14 from the IC surface.
According to the arrangement of FIG. 9, the Hall sensor IC 10 senses the movement and position of a ferromagnetic structure in form of the gear 18 by sensing and temporally evaluating the respective flux density of a magnetic field penetrating the Hall elements 12, 14. For this, the so-called backbias magnet 16 with South and North Poles (as drawn) is arranged on the backside of the sensor means 10.
In this procedure known in the prior art for sensing the movement or rotational direction of an indicator object, it is, however, disadvantageous that a stationary background signal (backbias signal), such as the field of a permanent magnet, is not canceled in the rotational speed recognition by means of magnetic field sensors. In other words, this means that in this procedure a relatively small modulation signal or useful signal generated by the teeth of the gear in the background field of the permanent magnet has to be evaluated, so that the useful signal is overlaid by a great static background signal. Moreover, it is disadvantageous in this procedure that the synchronism (matching) of the two magnetic field sensor elements (12, 14) has to be very good so that the offset difference of the output waveforms of both magnetic field sensor elements 12, 14 becomes small, in order to obtain reasonable measurement results.
According to the arrangement of FIG. 9, the Hall sensor IC 10 thus senses the movement and position of a ferromagnetic structure in shape of the gear 18 by sensing and temporally evaluating the flux density of a magnetic field. For this, the so-called backbias magnet 16 with South and North Poles (as drawn) is arranged on the backside of the sensor means 10.
A further procedure according to the prior art for the determination of the rotational direction and the rotation velocity of an indicator object, which works with three magnetic field sensor elements, is presented, for example, in the German patent (DE 19717364 C1). As illustrated in this patent, the output signals of the three magnetic field sensor elements are linked with each other into a directional signal so that a static background signal is faded out, wherein the phase shift of the directional signal to a differential signal of the two outer magnetic field sensor elements is used for the direction recognition. Here, now the zero crossing of the differential signal is used as sample time instant of the directional signal, namely once at a rising differential signal and once at a falling differential signal. The difference of the two sampled directional signals now determines the movement direction of the indicator object. In this procedure illustrated in the above-referenced German patent it is now to be noted that only “ideal” tooth distances (“ideal pitches”) are suitable for a sufficient performance of the sensor arrangement of three magnetic field sensor elements, because the evaluated directional signal becomes very small for greater tooth distances. It is spoken of “ideal pitches”, when the tooth distance matches the probe distance.
The procedure illustrated in the above-mentioned German patent is particularly based on generating, from this combination of three output signals of the three magnetic field sensor elements, a directional signal having a phase shift of 90° (π/2) to the differential signal. The differential signal is formed by a subtraction of the signals of the two outer magnetic field sensor elements. The directional signal is formed by the subtraction of the sum of the two sensor signals forming the differential signal and the double signal value of the output signal of the center magnetic field sensor element. The reason for the desired phase shift of 90° consists in obtaining an as great as possible directional signal at the zero crossing of the differential signal. Thus, the rotational direction or movement direction of the indicator object can be determined from the sign of the directional signal, when the directional signal is sampled at each zero crossing of the differential signal.
With reference to the procedure known in the prior art for the determination of the rotational direction of an indicator object and in particular the two procedures previously illustrated, it should be noted that only the vertical magnetic field component, i.e. the magnetic field component perpendicular to the surface of the magnetic field sensor with the sensor elements, is used or taken into account there.
With reference to the previously illustrated procedures according to the prior art for the determination of the rotational direction of an indicator object, it is also to be noted that, in the sensor arrangements used there, on the one hand the synchronism of the used magnetic field sensor elements has to be extremely good so that the offset differences of the magnetic field sensor elements are as small as possible, and also the positioning of the magnetic field sensor elements with reference to the indicator object the rotational direction of which is to be determined has to be made in an extremely exact manner, because otherwise too great and thus spurious phase errors, jitters, etc. occur in the output signal of this sensor arrangement.
The mean distance a (c.f. FIG. 9) of the sensor elements, according to the prior art, is to be adjusted so that the transitions or edges between the teeth (protruding portions) and the depressions or gaps (recessed portions) of the gear are sensed by the two sensor elements in succession in overlapping manner. With this, in a difference formation of the waveforms of the output signals of the two sensor elements, such a resulting differential signal may be obtained, which comprises signal peaks, when a tooth-gap-edge is moved past the sensor elements. The evaluation of this differential signal may however by very intensive due to overshoots in the waveform, because the output signal of the sensor elements is preferably to be processed further and rendered so that the course of the output signal substantially reproduces the profile of the indicator gear. Furthermore, in the previously used sensor arrangements, positioning inaccuracies between the indicator object and the sensor means impede the accurate evaluation of the sensor signal and cause phase noise, jitter, etc. in the sensor output signal.
Hence, it becomes obvious from the above-mentioned prior art that the evaluation of the sensor signals of the magnetic field sensor elements for the determination of the rotational direction of the indicator object is relatively intensive and also the determination of the velocity, the rotational angle or the rotational direction is not always sufficiently accurate.